fix: only accumulate when total agg type is SUM

For real this time :)

src/modules/pivotTable/PivotTableEngine.js

@@ -1095,7 +1095,7 @@ export class PivotTableEngine {
                     // accumulating other value types like text values does not make sense
                     if (
                         isCumulativeValueType(valueType) &&
-                        totalAggregationType === AGGREGATE_TYPE_AVERAGE
+                        totalAggregationType === AGGREGATE_TYPE_SUM
                     ) {
                         // initialise to 0 for cumulative types
                         // (||= is not transformed correctly in Babel with the current setup)